Think of what is in your car, & put it in a plane.
For example. A decent lock, including immobiliser, in the door. The average aircraft door can be opened, without damage, with a screwdriver.
Seats with adjustments for rake angle, squab angle, height, distance and cloth covers. You can't beat PVC seats for discomfort after a couple of hours in the sun.
Inertia reel seat belts.. . Tinted windscreens.. . A real handbrake the position of which clearly indicates on or off. (Cessna drivers will be nodding their heads in agreement here, I'm sure).. . More stowage space.